This application relates to devices and circuits based on magnetic tunnel junctions, including magnetic memory devices having magnetic tunnel junction cells that can be switched using a spin transfer torque effect.
A magnetic or magnetoresistive tunnel junction (MTJ) can include at least three layers: two ferromagnetic layers and a thin layer of a non-magnetic insulator as a barrier layer between the two ferromagnetic layers. The insulator material for the middle barrier layer is not electrically conductive and hence functions as a barrier between the two ferromagnetic layers. When the thickness of the insulator is sufficiently thin, e.g., a few nanometers or less, electrons in the two ferromagnetic layers can “penetrate” through the thin layer of the insulator due to a tunneling effect under a bias voltage applied to the two ferromagnetic layers across the barrier layer. The resistance to the electrical current across the MTJ structure varies with the relative direction of the magnetizations in the two ferromagnetic layers. When the magnetizations of the two ferromagnetic layers are parallel to each other in a parallel state (P), the resistance across the MTJ structure is at a minimum value RP. When the magnetizations of the two ferromagnetic layers are opposite to or anti-parallel with each other in an anti-parallel state (AP), the resistance across the MTJ is at a maximum value RAP. The magnitude of this effect can be characterized by a tunneling magnetoresistance (TMR) defined as (RAP−RP)/RP.
The relationship between the resistance to the current flowing across the MTJ and the relative magnetic direction between the two ferromagnetic layers in the TMR effect can be used for various applications including nonvolatile magnetic memory devices to store information in the magnetic state of the MTJ. Magnetic random access memory (MRAM) and other magnetic memory devices based on the TMR effect, for example, may be an alternative to and compete with electronic RAM devices in various applications. In such magnetic memory devices, one ferromagnetic layer is configured to have a fixed magnetic direction and the other ferromagnetic layer is a “free” layer whose magnetic direction can be changed to be either parallel or opposite to the fixed direction. Information is stored based on the relative magnetic direction of the two ferromagnetic layers on two sides of the barrier of the MTJ. For example, binary bits “1” and “0” may be recorded as the parallel (P) and anti-parallel (AP) orientations of the two ferromagnetic layers in the MTJ. Recording or writing a bit in the MTJ can be achieved by switching the magnetization direction of the free layer by, e.g., applying a writing magnetic field to the free layer or driving a current flowing across the MTJ based on the spin torque transfer effect.
The current switching based on the spin transfer effect may be used for high-density magnetic memory devices in part because the current required for current induced switching of the magnetization of the recording layers has been steadily decreasing as the device density grows following the scaling down rule compatible to semiconductor or CMOS technology evolution. The threshold of the spin-transfer switching current density Jc is now achievable at about 106 A/cm2 and can continue to decrease. This reduction of the spin-transfer switching current density Jc can lead to lower power consumption and smaller dimensions for the isolation transistor used in current switching MTJ-based MRAM devices. The current required for changing the magnetization of the free layer for recording data can be as small as 0.1 mA and is much lower than the current typically used for producing the switching magnetic field in the magnetic field switching MTJ. In addition, the degree of integration for MTJ cells can be approximately equal to that of DRAM cells and the write and readout times are expected to be comparable to those of SRAM cells.
Therefore, the MTJ cells based on the spin torque transfer switching can be used in high density memory devices and other applications.